I spent all my summers at our summer cottage with my grandparents when I was a kid. Among many things it was a great place to observe, and experience nature. Most of the things of course were quite stable or cyclical around the seasons. Things happened like they used to from year to year.
Over the years however I also had a chance to witness something rarer a few times. Something that only happened once, at least within my lifespan / while I was there observing them.
First one was the invasion of the pine sawfly larvae. Never seen them before (or even the sawfly itself) one year they were just there, filling all the pines, eating the needles and replacing them with themselves. As suddenly as they had appeared they also went away. Pines probably suffered some small damage due their feast, but besides that only the memory remained. There weren't even the massive amount of the sawflies later that summer which the amount of larvae would have suggested. Never seen one (larway or a fly) ever since.
This other time probably isn't that much of an invasion, just a weird gathering of ladybugs. One morning there were hundreds if not thousands of them floating at the shore. I've ever seen only a few of them at once beyond this one occasion. Maybe they were just washed away by the storm the previous night and now they were washing ashore.